NEPHELINITE

silica-poor (basic) lava that contains nepheline and pyroxene and is usually completely crystallized. Despite its wide geographic distribution and occasional extensive local development, it is a very rare rock. Known only from Tertiary strata (about 2,500,000 to 65,000,000 years in age), nephelinites are abundant in the Canary Islands, the Azores, the Cape Verde Islands, and Isla Fernando de Noronha off the coast of Brazil. The pyroxene in nephelinites may be either titanium-rich augite or aegirine; plagioclase, if present, is commonly labradorite. Varieties rich in leucite and haynite are well known. Biotite is characteristic in some types; amphibole is scarce. Accessory minerals may include sanidine, melilite, sodalite, perovskite, apatite, and chromite.